Radio communications apparatus with diversity

ABSTRACT

This invention relates to apparatus and a method for provision of diversity in reception in a communications system such as the GSM cellular radio system. A received call of low quality is received through a first of two antennas (15, 16) and through first of a plurality of receivers (10-14). A spare receiver that is not required for receipt of a call is identified and the spare receiver is switched to receive the call through the second antenna. The spare receiver is tuned to the frequency of the call of low quality and the calls are diversity combined through the first receiver and the spare receiver.

SUMMARY OF THE INVENTION

This invention relates to apparatus and a method for provision ofdiversity in reception for a communications system such as the GSMcellular radio system.

SUMMARY OF THE PRIOR ART

It is a problem in radio systems such as the GSM cellular radio systemoperating at frequencies of the order 900 MHz that a signal propagatesfrom a transmitting unit to a receiving unit over a number of paths and,at the receiving unit, it exhibits multi-path fading.

It is observed that if two or more receive antennas are provided,separated in distance, supplying signals to separate received channels,then the fading of the various channels is more or less independent,i.e. it is unlikely that all channels will fade together. Techniquesknown as diversity combining make use of several received signals toimprove the realised signal to noise ratio and hence improve the biterror rate. For example a typical dual diversity (2 channel) systemmight offer an improvement in signal to noise ratio (SNR) of 10 dB.

Providing diversity is not without substantial cost. It is not possiblemerely to combine the main and diversity signals because they caninterfere additively and destructively with equal probability. It isnecessary to duplicate the receive circuitry including the receiveamplifier and demodulator.

It would be desirable to provide diversity without the additional costof duplicated circuitry.

SUMMARY OF THE INVENTION

According to the present invention there is provided radiocommunications apparatus for receiving a plurality of calls comprising:a plurality of receivers tunable to a plurality of frequencies, firstand second antennas, switching means for switching the plurality ofreceivers to selected ones of the first and second antennas and controlmeans coupled to the plurality of receivers for tuning the receivers toreceive calls, the control means comprising: means for identifying areceived call of low quality received through the first of the antennasand through a first of the plurality of receivers, means for identifyinga spare receiver that is not required for receipt of a call, means forcausing the switching means to switch the spare receiver to receive thecall through the second antenna, means for tuning the spare receiver tothe frequency of the call of low quality and means for diversitycombining the calls through the first receiver and the spare receiver.

In this way, the control means utilize spare channel capacity todynamically allocate receiver units to the diversity antenna and in thisway there is provided a second source for the call identified as beingof low quality.

The invention takes advantage of the realisation that, in a given cellof a cellular radio system, only a proportion of the calls passingthrough a base station are of such low quality as to substantiallybenefit from diversity combining. The invention also recognizes that ata given time the receive units are rarely at 100% capacity, so thatthere is often spare channel capacity which can be utilized fordiversity purposes.

The invention provides the ability to add one or more extra receiveunits which can be dynamically shared for diversity purposes, ratherthan duplicating every receive unit.

In a time division multiplex system, a spare receive unit is identifiedfor each time slot (it may be a different unit in each time slot). As apreferred feature of the invention, the control means spread theon-going calls in time among different time slots so that there is, asfar as possible, spare channel capacity in each time slot. This sparechannel capacity can then be used for diversity.

As a further feature of the invention, the receive antenna may besectorized and the diversity antenna may be an adjacent sector of thereceive antenna. This feature recognizes that the lobes of adjacentsectors of a sectorized antenna usually overlap to a degree, so that itis frequently the case that the signal from a mobile unit can bereceived at two radially separated sectors of a sectorized antenna. Thisfeature avoids the necessity to have a second physically separatedantenna, thereby further reducing cost.

The invention has the advantage of providing diversity by more fullyutilizing existing equipment, merely by appropriate switching andappropriate software.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of receive circuitry of a base station inaccordance with the present invention;

FIG. 2 is a representation of calls passing through receive channelunits (RCUs) of FIG. 1 on different time slots;

FIG. 3 is a block diagram of receive circuitry of a base station inaccordance with second and third embodiment of the invention; and

FIG. 4 is a diagram of a sectorized cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the circuitry comprises five receive channel units(RCUs) a numbered 10 to 14. The invention is equally applicable to anynumber of RCU's. Each of these units comprises amplifier circuitry,tuner circuitry and demodulator circuitry (not shown) and the output ofeach of these units is 13 Kbit encoded speech for passing to a vocoder.Each RCU is tunable to a selection of frequencies, which are thefrequencies allocated to the cell served by the base station and areselected from the total set of frequencies for the cellular radiosystem.

Also shown in FIG. 1 are two antennas, a "main" antenna 15 and a"diversity" antenna 16. These are connected to splitters 17 and 18respectively, each of which splits the received signal from its antennainto 5 separate signals. The splitters 17 and 18 are connected to theRCUs 10 to 14 through a switch matrix 19. The switch matrix 19 isarranged to connect any one or more of the RCUs 10 to 14 to either thesplitter 17 or the splitter 18, on a per timeslot basis.

The RCUs 10 to 14 are under the control of a controller 20 in the formof a microprocessor. The RCUs in turn control the switch matrix 19 byusing the coaxial cables from the switch matrix 19 to pass 1200 bauddata back to the switch matrix 19 multiplexed with the r.f. signalspassing in the other direction. As an alternative to receiving controldata from the RCUs, control can come directly from the controller 20over a local area network to which the switch matrix 19 can beconnected.

The controller 20 controls the setting up of channels and the allocationof time slots to receive (and transmit) calls.

The manner in which the controller 20 controls the allocation offrequencies and time slots to set up calls is illustrated in FIG. 2. Inthis figure, eight time slots are shown in a TDMA arrangement and fivereceive channel units are illustrated. A "channel" is a RCU tuned to aparticular frequency on a particular time slot. A single frequency canyield 8 channels and a base station having five RCUs can pass 40channels. As shown in FIG. 2, on the first time slot shown (T1) RCU 10is tuned to frequency F1, RCU 11, 12 and 13 are tuned to frequencies F2,F3 and F4 respectively and, in this example and in accordance with theinvention at least in its preferred embodiment, RCU 14 is tuned tofrequency F1. In this manner, RCU 14 is being used as a diversityreceive channel unit in parallel with RCU 10.

Referring again to FIG. 1, RCU 10 controls the switch matrix 19 toconnect RCU 10 to splitter 17 and RCU 14 controls the matrix to connectRCU 14 to splitter 18, so that these RCUs receive signals from the mainantenna 15 and the diversity antenna 16 respectively. The same call isbeing processed separately through two separate RCUs and the resultantdata stream can be diversity combined using selection diversity, maximumratio diversity or equal gain diversity.

As an example of selection diversity the 13 Kbit encoded speech ispassed to an equalizer and an error corrector (not shown), and the errorcorrector provides an indication of bit error rate. A selection is madebetween the two bit streams so as to select the bit stream having thelower bit error rate. Other arrangements can be provided to give maximumratio diversity or equal gain diversity.

The manner in which the controller 20 selects a particular RCU forconnection to the diversity antenna is as follows. Each RCU 10 to 14provides two measurements of signal quality designated as Rxlev andRxqual, which represent the received signal strength indication (RSSI)and the bit error rate (BER). In a given time slot the controller 20measures these two indications for each call passing through the RCU. Itdetermines whether one or both of these indicators falls below arespective threshold. Where this is the case for a given call, thecontroller 20 determines whether there is a spare RCU for which there isno call being received on that time slot and, where a spare RCU isavailable, it causes that RCU to be tuned to the same frequency as thelow quality signal and applies diversity to that particular call.

In a given cell, only a small proportion of units, especially those onthe boundary of the cell, will, on average, experience poor signalquality. Using the Rxlev and Rxqual indicators, it is possible toidentify a signal that is subject to multi-path fading and, by providingdiversity, the additional antenna can enhance the signal and inparticular can smooth out troughs in the signal.

The controller 20 maintains a matrix, as shown in FIG. 2, correlatingcalls (i.e. frequencies assigned to individual mobiles) on each timeslot. It is a preferred feature of the invention that the controller 20manages the allocation of time slots as far as possible to maintain aspare RCU in each time slot. This is illustrated in FIG. 2 where each ofthe time slots T1 to T8 has only 4 out of a possible 5 calls. Clearlythis is not possible when the cell is fully loaded (i.e. with 40 callspassing through at a given time). In setting up a new call, thecontroller 20 chooses, if possible, a time slot that is 3/5 loaded orless, so that the time slot will still not be fully loaded afterallocation of the new call. This means that there is still spare channelcapacity on that time slot for diversity purposes. In situation shown inFIG. 2, a new call could be set up on time slot T7 without fully loadingthat time slot. The system would still be managed such that each timeslot has a spare RCU which is not passing any call, so that the spareRCU can be used for diversity. It is not necessarily the same RCU foreach time slot.

Similarly, when a call terminates, a RCU on a time slot becomes free(unless there is another mobile queuing to make a call). In thissituation, the controller 20 has the ability to move a call on to thattime slot in order to make spare capacity available on a different timeslot. This is achieved by an intra-cell handover in which the basestation instructs the mobile to change its time slot with or without achange of frequency and simultaneously the base station and mobilechange to the new time slot (and frequency if necessary).

Where there is more than one unit needing diversity on a given timeslot, the control 20 can arrange for there to be more than one spare RCUavailable on that time slot. Conversely, the controller 20 can spreadout the "problematic" calls so that they do not all occur in the sametime slot, thereby spreading out the spare capacity to be allocatedaccordingly.

Diversity can be applied in this way as soon as a call commences, orduring a call, when it begins to fade or when a spare RCU becomesavailable. Diversity may be stopped when the spare RCU is required toset up a new call.

The above arrangement has been described with reference to an"omnicell", i.e. a cell having a 360° main antenna and a 360° diversityantenna.

It is a preferred feature of the preset invention that the first andsecond antennas are adjacent antennas of a sectorized antenna. Thisfeature has the advantage of making greater use of equipment inherent ina sectorized cell and avoiding the need for additional expensiveantennas.

Antennas are generally sectorized in either three sectors of 120° or sixsectors of 60° (see, for example, U.S. Pat. No. 4,128,740 of Graziano,assigned to Motorola Inc.)

In the case of a three-sector antenna, it is generally necessary toprovide three diversity antennas generally covering the same sectors asthe three main antennas. This arrangement is illustrated in FIG. 3. Inthis figure, there is shown six antennas 30 to 35. In the case of 120°sectorized cells, antennas 30, 32 and 34 serve the three sectors of thecell and antennas 31, 33 and 35 are positioned above or below antennas30, 32 and 34 to provide diversity in each of the sectors. The antennasare connected through pre-selectors 40 and pre-amplifiers 41 to receivematrixes 42 and 43. Matrix 42 is connected to the main antennas 30, 32and 34 and matrix 43 is connected to the diversity antennas 31, 33 and35. Each matrix is arranged to switch any one of its three antennas toany one of six output ports. Five of these output ports are connected toRCU as shown in FIG. 1 and the sixth output port is for test purposesonly.

In the case of a 60° sectorized cell, the same arrangement as isillustrated in FIG. 3 can be used, but in this case each of the antennas30 to 35 covers a different sector of the cell. In this arrangement,there are no diversity antennas. The operation will be described withreference to FIG. 4, which is an illustration of a 60° sectorized cell.

60° sectorized cells are normally only used in areas of high traffic, inwhich a single rack of five RCUs is insufficient to serve the cell, andnormally two such racks are provided. Thus the cell can handle 80 callssimultaneously. One rack of RCUs serves, for example sectors 1, 2 and 3of FIG. 4 and the other rack serves sectors 4, 5 and 6.

It is a feature of sectorized cells that the antennas do not giveexactly 60° coverage, but generally give substantially more than 60°coverage. Thus, there is a large area of overlap between adjacentsectors. Where a mobile is operating at the edge of one sector, it maybe susceptible to fading. At the same time the present inventionrecognizes that a mobile operating in this area may provide a reasonablesignal to the adjacent sector and these signals can be diversitycombined through separate RCUs. Thus, for example, where sector 2 ofFIG. 4 is serving a mobile and the Rxlev or Rxqual indication of thatmobile falls below a preset threshold, the processor 20 decides to applydiversity to that call and sets up a parallel channel through sector 1.The signals through sectors 1 and 2 are processed through parallel RCUs(assuming that a "spare" RCU is available) and a diversity decision istaken using the demodulated signals. If it emerges that the signalthrough sector 1 does not provide an improvement in this way, thecontroller 20 causes the "spare" RCU to switch to the antenna servingsector 3 on the next occurrence of a time slot on the channel inquestion.

A diversity signal will generally not provide improvement if it is lessthan 20 dB below the main signal. Thus, if either of the sectorsadjacent sector 2, in which the mobile is located, provides a signalfrom that mobile within 20 dB of the main signal, diversity can be usedto improve the main signal. No additional antennas or RCUs have had tobe provided. Expressing this in terms of SNR, where a call on a channelin a sector is determined as having a SNR below a threshold, and wherethe same call is being received through a second antenna at a SNR within20 dB of the first value, diversity can usefully be applied.

It has been explained that a rack of five RCUs generally serves threesectors of a 60° sectorized cell. Thus a rack of five RCUs will servesectors 1, 2 and 3, with diversity being provided from sectors 6 and 4.Sector 5 is not connected and accordingly, the corresponding input tothe matrix 42 or 43 is simply terminated. Likewise, the other rack offive RCUs Serving sectors 4, 5 and 6 has its input terminatedcorresponding to the antenna from sector 2.

We claim:
 1. Radio communications apparatus for receiving a plurality ofcalls comprising:a plurality of receivers tunable to a plurality offrequencies, first and second antennas, switching means for switchingthe plurality of receivers to selected ones of the first and secondantennas and control means coupled to the plurality of receivers fortuning the receivers to receive calls, the control meanscomprising:means for identifying a received call of low quality receivedthrough the first of the antennas and through a first of the pluralityof receivers, means for identifying a spare receiver that is notrequired for receipt of a call, means for causing the switching means toswitch the spare receiver to receive the call through the secondantenna, means for tuning the spare receiver to the frequency of thecall of low quality and means for diversity combining the calls throughthe first receiver and the spare receiver.
 2. Apparatus according toclaim 1 for receiving a plurality of calls separated over different timeslots in a time-division multiple-access communications system whereinthe control means are arranged to cause transmission of messages toremote units, to control those remote units for setting up of calls onselected time slots and wherein the control means are arranged todistribute the setting up of calls over the different time slots sothat, during below-saturation operation, spare receivers are availableon the same time slots as calls of low quality.
 3. Apparatus accordingto claim 2 wherein the control means comprise means for terminating acall on a time slot, means for identifying that a corresponding receiverhas consequently become spare and means for matching that spare receiverwith another call of low quality on the same time slot.
 4. Apparatusaccording to claim 3 wherein the means for matching comprises means foridentifying a call of satisfactory quality on the same time slot as thecall of low quality and means for transmitting a message to acorresponding remote unit to cause that call to be moved in time to thetime slot on which the terminated call resided, thereby creating a sparereceiver on the time slot of the call of low quality.
 5. Apparatusaccording to claim 1 wherein the first and second antennas aredirectional antennas extending over the same sector of a sectorizedantenna.
 6. Apparatus according to claim 1 wherein the first and secondantennas are adjacent directional antennas of a sectorized antenna. 7.Apparatus according to claim 1 wherein the means for identifying a callof low quality comprises means for determining that the call has asignal-to-noise ratio below a predetermined threshold.
 8. Apparatusaccording to claim 1 wherein the means for identifying a call of lowquality comprises means for determining that the call has a bit errorrate below a predetermined threshold.
 9. Apparatus according to claim 1wherein the means for diversity combining the calls only combines thecalls when they are determined as having signal-to-noise ratios within apredetermined range of each other.
 10. A method of reception of a callcomprising the steps of:providing a plurality of receivers tunable to aplurality of frequencies; providing first and second antennas; providingswitching means for switching the plurality of receivers to selectedones of the first and second antennas; controlling the plurality ofreceivers for tuning the receivers to set up channels to receive calls;identifying a received call of low quality received through the first ofthe antennas and through a first of the plurality of receivers;identifying a spare receiver that is not required for receipt of a call;causing the switching means to switch the spare receiver to receive thecall through the second antenna; tuning the spare receiver to thefrequency of the call of low quality; diversity combining the callsthrough the first receiver and the spare receiver.